The present invention pertains to a thread tension control for sewing machines which in addition to their main transport device for advancing a workpiece that is operatively associated with the stitching instrumentalities, they are also provided with an auxiliary transport device which is disposed in alignment with and downstream of said stitching instrumentalities.
These auxiliary transport devices serve to assist in the advance of a workpiece and at the same time maintain it in a stretched state so as to oppose the tension to which the threads are subjected by the stitching instrumentalities during stitch formation thereby preventing such tension from causing a puckering of the fabric defining the workpiece during the formation of each stitch.
When forming a seam with stitches of the double chain type and the stitches extend parallel with and adjacent the edge of a workpiece, the tension to which the threads are subjected while being manipulated to form a stitch create a condition which causes bunching, puckering or in other words a reduction in the intended length of said workpiece and is responsible in forming stitches therein of a length less than that desired. In practice such bunching or puckering is more prevalent and pronounced in a workpiece when there is a decrease in the consistency thereof and when the spacing between the successive penetrations of the needle into the workpiece is caused to be increased.
This undesirable condition of puckering of a workpiece during the stitching cycle is overcome by the auxiliary transport device which serves to draw said workpiece toward it by an amount that is equal to or slightly greater than the amount being advanced by the main transport device of the sewing machine.
Control of the workpiece in this manner provides a means of eliminating the condition of puckering by opposing the tension of the thread so that said tension is effective in completing each stitch that is formed with the desired degree of tightness.
The auxiliary transport device performs its intended function satisfactorily during the seaming cycle; however, after the last stitch is formed in the workpiece and it is advanced beyond the main transport device and the stitching instrumentalities, the chain of stitches which continues to be formed prior to the start of seaming on the following workpiece is subjected to the full pulling force of the auxiliary transport device until said following workpiece has been advanced a sufficient distance to be acted upon by said auxiliary transport device. With a condition of this sort, the pulling force which the auxiliary transport device applies to the chain is such that it is possible to easily overstress the threads, for said auxiliary transport device is advancing or pulling the chain at a slightly faster rate than the stitching instrumentalities are able to manipulate the threads being withdrawn from a supply spool via a tensioning device to form said chain.
Subjecting the chain to a pulling force as described above is considered a very serious and undesirable condition and is especially troublesome when the threads utilized to form the stitches of a chain have no elasticity, for during this pulling force the forming of each stitch requires that the looper perform its function of taking a loop of thread from the needle as the latter is being withdrawn. Additionally, the fact that the looper is caused to travel in a direction different from the direction of pull of the auxiliary transport device further aggravates this condition.
It is not uncommon for the amount of pulling force provided by the auxiliary transport device to become greater than the strength of the threads so as to cause breakage thereof.
After linkage of the threads has been made to form the initial stitches in a workpiece which serves as a support for said threads, the pulling force changes gradually and in an amount which corresponds to the amount of fabric forming the workpiece that is being advanced by the auxiliary transport device. When the workpiece is no longer in a position to oppose the pulling force of the auxiliary transport device, the threads are subjected to the full pulling force thereof.
Breakage of a chain of stitches by the pulling force of the auxiliary transport device interrupts the continuity of the sewing operation on the next following workpiece for there is no means for drawing or guiding the same, without manual intervention, to a position to be acted upon by the auxiliary transport device. This undesireable condition is more troublesome in sewing machines where the main transport device is formed by counter-rotating disc members and the auxiliary transport device by two counter-rotating rollers that are intermittently rotated in synchronization with said disc members and where the axes of rotation of the latter are vertically disposed so that the workpiece is also supported vertically as it is caused to be advanced therebetween.
The main object of the present invention is to improve the performance of the sewing machine by eliminating the problem of chain breakage and during the absence of a workpiece to effect a desired closing of each stitch forming the chain.
To eliminate the problem of chain breakage the invention provides a means for reducing the tension on the threads, when a workpiece is absent during machine operation, down to a value which is sufficient to adequately effect linkage of the threads so as to form a chain. Additionally, the invention also includes a means for returning the thread tension to a normal operating value when a workpiece is present in order to obtain the desired closing of the stitches being formed in said workpiece.